It is possible to disable Firefox Australis UI and restore initial interface. Australis is new interface for Firefox that has currently landed in Nightly and available for testing. This will be available worldwide with Firefox 28 release in April 2014. Some already hated the new interface, some liked it. If you’ve enough reasons to revert/get back to old Firefox interface good news for those is that an add-on is available on AMO which does exactly that.

Classic Theme Restorer extension available for Mac, Windows and Linux restores above said features- restores squared tabs, appmenu, add-ons bar, small icon view and some old buttons. After installing this extension and restart of browser, you can see old Firefox interface with squared tabs by default. Add-on bar and Tabs not on top and other changes to enable are available in add-on options.